Questions about Wiko Lenny 3

I remember that the SD card can be formatted either as a memory extension or as portable memory (I don't remember the exact name). The SD card can only be read as portable memory.

Well, just like on PC you have to distinguish between storage memory and main memory (RAM). 16GB are for storage, where also Android is installed. The 1GB is RAM, used for computing and handling all the different tasks currently in use on your phone.